In a conventional air passage switching door for an air conditioner described in U.S. Pat. No. 6,047,951 (corresponding to JP-A-11-180129), a seal member that is made of an elastomer, a rubber or the like, is provided on an outer peripheral portion of a door body, and is press-fitted to a sealing surface of a case having an air passage in order to switch the air passage. In the air passage switching door, the door body is made of resin to have a high rigidity (i.e., inelastic property). Further, a rotation shaft is formed integrally with the door body at a center in a lateral direction. Generally, the seal member made of a rubber material is inserted into a mold at a preset position in the mold, and thereafter a resin material is injected into the mold, so that the outer peripheral portion of the door body is connected to the seal member.
However, when the seal member press-contacts the seal surface for closing the air passage, peeling force (i.e., separation force) works to the outer peripheral portion of the door body. Adhesion strength is required between the seal member and the door body at the outer peripheral portion, so that the outer peripheral portion of the door body is not separated from the seal member due to the peering force working on the outer peripheral portion in a shearing direction (see “A” direction in FIG. 3). Thus, when the adhesion strength is low, the seal member is separated from the door body.